If this boat is an I/O with the outdrive removed you'll have a rather large hole in the stern where there used to be an outdrive, simply attaching an engine to the stern of this is not a good idea.
I/O style boats are built different than non I/O boats the stern plates on non I/O boats are braced and strengthened to support the motor, while on an I/O style the outdrive is part of the bracing, the gimble acts as a support. You might want to rethink this one before somebodies forced to go down with the ship.
You cant find a cheap fishing boat on CL?